Prior to the present invention, the liquid spray application of coatings, such as lacquers, enamels and varnishes, was effected solely through the use of organic solvents as viscosity reduction diluents. However, because of increased environmental concern, efforts have been directed to reducing the pollution resulting from painting and finishing operations. For this reason there has been a great deal of emphasis placed on the development of new coatings technologies which diminish the emission of organic solvent vapors. A number of technologies have emerged as having met most but not all of the performance and application requirements, and at the same time meeting emission requirements and regulations. They are: (a) powder coatings, (b) water-borne dispersions, (c) water-borne solutions, (d) non-aqueous dispersions, and (e) high solids coatings. Each of these technologies has been employed in certain applications and each has found a niche in a particular industry. However, at the present time, none has provided the performance and application properties that were initially expected.
Powder coatings, for example, while providing ultra low emission of organic vapors, are characterized by poor gloss or good gloss with heavy orange peel, poor definition of image gloss (DOI), and poor film uniformity. Pigmentation of powder coatings is often difficult, requiring at times milling and extrusion of the polymer-pigment composite mixture followed by cryogenic grinding. In addition, changing colors of the coating often requires its complete cleaning, because of dust contamination of the application equipment and finishing area.
Water borne coatings cannot be applied under conditions of high relative humidity without serious coating defects. These defects result from the fact that under conditions of high humidity, water evaporates more slowly than the organic cosolvents of the coalescing aid, and as might be expected in the case of aqueous dispersions, the loss of the organic cosolvent/coalescing aid interferes with film formation. Poor gloss, poor uniformity, and pin holes unfortunately often result. Additionally, water borne coatings are not as resistant to corrosive environments as are the more conventional solvent borne coatings.
Coatings applied with organic solvents at high solids levels avoid many of the pitfalls of powder and waterborne coatings. However, in these systems the molecular weight of the polymer has been decreased and reactive functionality has been incorporated therein so that further polymerization and crosslinking can take place after the coating has been applied. It has been hoped that this type of coating will meet the ever-increasing regulatory requirements and yet meet the most exacting coatings performance demands. However, there is a limit as to the ability of this technology to meet the performance requirement of a commercial coating operation. Present high solids systems have difficulty in application to vertical surfaces without running and sagging of the coating. Often they are also prone to cratering and pin holing of the coating. If they possess good reactivity, they often have poor shelf and pot life. However, if they have adequate shelf stability, they cure and/or crosslink slowly or require high temperature to effect an adequate coating of the substrate.
U.S. Pat. No. 4,582,731 (Smith) discloses a method and apparatus for the deposition of thin films and the formation of powder coatings through the molecular spray of solutes dissolved in organic and supercritical fluid solvents. The molecular sprays disclosed in the Smith patent are composed of droplets having diameters of about 30 Anstroms.
These droplets are more than 10.sup.6 to 10.sup.9 less massive than the droplets formed in conventional application methods which Smith refers to as "liquid spray" applications. The disclosed method of depositing thin films also seeks to minimize, and preferably eliminate, the presence of solvent within the film deposited upon a substrate. This result is preferably accomplished through the maintenance of reduced pressure in the spray environment. However, low solvent concentration within the deposited film leads to the same problems encountered through the use of high solids coatings. The maintenance of reduced pressures is also not feasible for most commercial coating applications. Furthermore, the spray method disclosed by Smith utilizes very high solvent to solute ratios, thereby requiring undesirably high solvent usage and requiring prohibitively long application times in order to achieve coatings having sufficient thicknesses to impart the desired durability to the coating.
Clearly, what is needed is an environmentally safe, non polluting diluent that can be used to thin very highly viscous polymer and coatings compositions to liquid spray application consistency. Such a diluent would allow utilization of the best aspects of organic solvent borne coatings applications and performance while reducing the environmental concerns to an acceptable level. Such a coating system could meet the requirements of shop and field applied liquid spray coatings as well as factory applied finishes and still be in compliance with environmental regulations.
It is accordingly an object of the present invention to demonstrate the use of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent borne and/or highly viscous non aqueous dispersions coatings compositions to dilute these compositions to application viscosity required for liquid spray techniques.
A further object of the invention is to demonstrate that the method is generally applicable to all organic solvent borne coatings systems.
These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein set forth.